In clinker production, the proportion of alternative fuels is increasing greatly, which in turn results in an increasing input of chlorine into the production process. On account of its evaporation and condensation temperatures, chlorine forms an internal circulation in the preheater/kiln system. This circulation is relieved by way of a bypass system, in which part of the kiln gas is removed at the kiln inlet. Such a bypass may also be used for discharging SO2 and alkalis. Increasing use of alternative fuels in some cases requires high bypass rates of the order of up to 10% and more. The bypass offgas is branched off at the furnace inlet at temperatures of, for example, more than 600° C. or of more than 1000° C., depending on the application of the kiln system, and shortly after being drawn off is quenched, for example in the so-called bypass bell or a heat exchanger, to 500 to 150° C., preferably 400 to 200° C., most preferably 370 to 240° C. At these temperatures, the gas can be fed to an electrostatic or fabric filter for dust separation. On account of their condensation temperatures, gaseous chlorine and sulfur are therefore also incorporated in the bypass filter.
According to DD 274 022 A1, an alkali- and chlorine-containing bypass gas is branched off from the clinker burning process at temperatures of 1050 to 1250° C. and cooled down by supplying fresh air to 700 to 1050° C. After that, 50 to 70% of the dust contained in the gas stream is separated from the gas stream and fed back to the burning process. As a result of the low degree of separation in the separation of the dust from the gas stream, only the low-alkali, coarse-grained dust fractions are separated out of the gas stream and returned to the burning process. At temperatures of 700 to 1050° C., the alkalis are partly still volatile or are only precipitated on the smallest grain fractions of the dust. After further cooling, these harmful substances are then separated from the gas stream.
Nitrogen oxides are produced during clinker production on account of the high temperatures in the kiln due to the oxidation of nitrogen from the combustion air. This is typically alleviated by a staged combustion and the injection of an ammonia-containing reducing agent in the region of the calciner or kiln inlet. Nitrogen oxides in the kiln offgas are also removed by means of catalytic converters in the downstream offgas system.
If there are high requirements for the nitrogen oxide concentration in the clean gas, such as for example the 200 mg/Nm3 stipulated by the 17th German Federal Emission Control Ordinance for the co-combustion of waste in cement works, for many plants the nitrogen concentration in an averaged offgas concentration of bypass and kiln gas cannot be made to comply. An improvement can be achieved by denitration of the bypass gas. However, the SNCR reaction can only take place unsatisfactorily on account of the low dwell times at sufficiently high temperatures in the bypass gas. Classic SCR catalytic converters have a comparatively high system expenditure and can only respond inadequately to fluctuating dust loads, such as occur in the bypass gas.
Accordingly there is a need for an improved method and system for reducing harmful gaseous substances in the bypass gas of the cement or mineral industry.